Mixing, or blending, machines having one or more horizontal agitator shafts positioned in an elongated tub are in common use in the blending of particulated food products such as different mixtures of diced or ground meat, poultry, vegetables, sauces and the like. The most commonly used machine is a twin shaft mixing machine, wherein two horizontal agitator shafts are mounted in a tub parallel to each other. The agitators mounted on the shafts come in many designs, with the most common being a ribbon agitator wherein a spiral ribbon of steel is mounted on each shaft by spokes extending radially from the shafts.
As the agitator shafts rotate, the spiral ribbons push through the product causing it to move in rotating column with the agitator, and, because the ribbons are spiral, to move slowly in a direction parallel to the agitator shafts, i.e. from end-to-end in the tub. Typically, the agitators are rotated in opposite directions so that the rotating columns of the product are moved in opposite end-to-end directions in the tub by the agitators, with the product being continuously folded into the center of the tub and mixed by the counter-rotating agitators.
An example of such a mixing machine is that shown in U.S. Pat. No. 4,733,607, issued Mar. 29, 1988 to Leonard J. Star and Jess J. Tapscott. In this patent, the apparatus also includes a steam jacket surrounding the tub so that the product can be cooked as the agitators mix and blend the product together. Also in the patent, the spiral ribbons have scrapers mounted thereon for scraping the trough walls to keep the product from sticking on the hot cooking surfaces. Ribbon agitator machines used for cold mixing will not have a steam jacket, nor will the scrapers shown in the above patent be required.
Historically, difficult-to-mix products have been mixed on a batch basis in batch operations machines as shown in the above mentioned U.S. Pat. No. 4,733,607. The reason for this is batch systems control all particles of the batch until they evenly mix together. Sticky products such as ground beef products, thick vegetable or fruit slurries do not mix evenly. Pockets of unmixed product remain until very late in the mixing cycle. Two horizontal agitators in a batch mixer have been employed to break up these pockets and evenly distribute all the different ingredients throughout the batch. The horizontal ribbon or paddle agitators fold the ingredients together from one agitator to the other. The length of the mixing cycle is determined by how long it takes to break up the unmixed pockets and evenly distribute all the particles.
Cooking of thick, viscous products is even more difficult. The particles close to the heat exchange surfaces heat up first and must be mechanically pushed away from the heated walls and evenly distributed throughout the batch so that uncooked, cooler particle will come in contact with the heated surfaces of the cooker. If the agitators do not continuously mix the batch during heat-up, some particles will overcook and destroy the flavor of the whole batch.
An additional challenge when cooking some products is that the viscosity of the product will change with temperature. Some products thin out, making it difficult to control the movement of the particles in cold spots in the batch. Some products with starch thicken up with the temperature increase.
All of the above variables have prevented food processing machine designers from seriously considering continuous mixing and cooking of the viscous products. Continuous mixing and cooking, however, is very desirable. Continuous systems are more labor-efficient and the equipment is more cost effective since more production is possible with the same capital outlay. Usually the utility operating costs are less also. Whenever possible, a food processor will install a continuous system over a batch system. Unfortunately, virtually all mixing and cooking of viscous, sticky or variable viscosity products are carried out by batch processing. This means a processor must break out of this continuous flow and batch all mixing elements of his production line and then go back to a continuous flow for packaging. This change in product flow is disruptive and expensive.
Efforts have been made to produce a continuous mixer using the same general concept of two elongated spiral ribbon agitators in an elongated mixing tub. For example, FIGS. 4 and 5 of U.S. Pat. No. 4,941,132, issued July 10, 1990, to Darrell C. Horn and John M. Lennox, III, disclose a continuous mixer having two counter-rotating agitators, oppositely wound on their shafts. Rotation of the shafts in their forward directions will cause the products in each trough of the tub to fold and mix with each other centrally of the tub. At the same time rotation of both shafts in this forward direction will urge the columns of products in both troughs to move together towards the discharge end of the tub. The direction of rotation of the shafts is periodically reversed, with the length of time of reverse rotation being less than the length of time of forward rotation so that the total length of time for the product to be moved incrementally from the inlet end of the tub to the discharge end of the tub to the discharge end of the tub can be regulated. The ingredients of the product are continuously fed into the inlet end of the tub and continuously removed from the discharge end.
Such a continuous mixer can, however, only be used with ingredients that are easy to mix, which do not flow and which do not require the end-to-end mixing of columns moving in opposite directions (i.e. as in batch mixers).